A DMOS (Double diffused MOS) transistor is a type of MOSFET (Metal Oxide Semiconductor Field Effect Transistor) that uses two sequential diffusion steps aligned to the same edge to form the channel region of the transistor. DMOS transistors are often high voltage, high current devices, used either as discrete transistors or as components in power integrated circuits. DMOS transistors can provide high current per unit area with a low forward voltage drop.
One particular type of DMOS transistor is a so-called trench DMOS transistor in which the channel is present on the sidewall of a trench, with the gate formed in the trench, which extends from the source towards the drain. The trench, which is lined with a thin oxide layer and filled with polysilicon, allows less constricted current flow than the planar gate DMOS transistor structure and thereby provides lower values of specific on-resistance.
Dual gate trench MOSFETs have been developed and manufactured. US publication No. 2006/0273386 discloses a method of making a shield gate field effect transistor including a body region of a first conductivity type over a semiconductor region of a second conductivity type. A gate trench extends through the body region and terminates within the semiconductor region. At least one conductive shield electrode is disposed in the gate trench. The shield electrode is connected to the source voltage and shields the gate electrode from the drain to reduce the gate-drain capacitance (Cgd) and to improve the breakdown voltage. A gate electrode is disposed in the gate trench over but insulated from the at least one conductive shield electrode. A shield dielectric layer insulates the at lease one conductive shield electrode from the semiconductor region. A gate dielectric layer insulates the gate electrode from the body region. The shield dielectric layer is formed such that it flares out and extends directly under the body region.
However, a conventional method of making such a shield gate field effect transistor requires a six to eight masks process that is expensive and time consuming.
It is within this context that embodiments of the present invention arise.